The known conventional method of manufacturing diodes, insulated gate bipolar transistors (hereinafter referred to as “IGBTs”) and such power semiconductor devices uses a thick silicon wafer. At the final stage of the manufacturing process, the thick silicon wafer is ground thin and thereafter etched to form a predetermined final thickness. Thereafter, implanted ions are activated by an activation heat treatment. See for instance, published PCT application WO 00/04596 or. Japanese published patent application 2002-520885. Recently, the manufacturing process described above has been mainly employed.
Since electrodes have been formed already on the semiconductor wafer surface opposite to the surface thereof ground thin, it is necessary to conduct the activation heat treatment below the melting point of the electrode material, e.g., below 450° C. for aluminum. Due to the activation at a lower temperature, the implanted impurity ions are liable to be activated insufficiently. The other known method for obviating this problem includes forming a p-type anode layer and an anode electrode of a diode on one major surface of an FZ wafer, grinding the opposite major surface of the FZ wafer, implanting ions such as phosphorus ions and arsenic ions from the ground major surface, and activating the implanted ions by irradiating a laser beam. See for instance, U.S. Pat. No. 6,759,301. The FZ wafer is a wafer cut out from an ingot grown by a floating zone method and is less expensive than the epitaxial wafer.
It has been reported that it is preferable for the forward voltage temperature coefficient of a diode to be positive. See for instance Michio NEMOTO, et al, An Advanced FWD Design Concept with Superior Soft Reverse Recovery Characteristics, (USA) ISPSD. Proceedings, (2000), pp. 119-122. For making it possible to provide an IGBT module with a high current capacity (e.g., 500 A or higher), it is sometimes necessary to connect the IGBT chips and the freewheeling diode chips (hereinafter referred to as the “FWD chips”) parallel to each other. Since the positive temperature coefficient of the diode forward voltage facilitates preventing the current from localizing to a specific chip in the above-described configuration, current balance is maintained stably between the chips.
For adjusting the forward voltage temperature coefficient to be positive, it is a precondition that the minority carrier lifetime (the doped lifetime killer amount) be controlled by electron beam irradiation. See for instance Japanese published patent application 2001-177114. The platinum diffusion, which is a typical technique for controlling the minority carrier lifetime, provides the diode forward voltage with a negative temperature coefficient since the platinum energy level is shallow.
The NEMOTO, et al publication points out that the reverse recovery of the diode is liable to be the so-called hard recovery, when the electron beam irradiation is employed, and oscillations are liable to occur during the reverse recovery, since the crystal defects introduced in the semiconductor substrate by the electron beam irradiation distribute uniformly in the thickness direction of the semiconductor substrate, namely in the electron beam irradiation direction.
Therefore, for manufacturing a device with the forward voltage thereof exhibiting a positive temperature coefficient, it is necessary not only to control the minority carrier lifetime by electron beam irradiation but also to realize a crystal defect distribution suitable for realizing soft recovery. One of the known methods for realizing the preferable crystal defect distribution makes the impurity concentration in an n-type drift layer peak at the center thereof and decrease toward a p-type anode layer and an n-type cathode layer. See for instance. Japanese published patent application 2003-318412.
The method disclosed in the Japanese published application above that uses an epitaxial wafer, however, is not suitable for using the inexpensive FZ wafer, which has been used mainly in these days. Therefore, there still remains a need to develop a new method that allows use of an inexpensive FZ wafer, controlling the minority carrier lifetime by electron beam irradiation, and still obtaining soft recovery characteristics. The present invention addresses this need.